Most printing applications are designed for printing onto white substrates for document production or photo printing. For such applications, black inks are primarily used in combination with three or more colored inks. Formulating stable black inks and colored inks is well understood. For instance, it is known that molecular dyes will generally not exhibit settling over time if they are soluble in the carrier. In addition, pigment-based black inks generally utilize particle sizes small enough to be suspended in low viscosity carriers and take advantage of both Stokes' Law and Brownian motion to keep the particles suspended. However, these inks exhibit very low opacity, as dyes are transparent by nature and pigmented-based black inks use particle sizes that are small enough to be suspended but are much smaller than the particle sizes required to scatter visible light. For opacifying purposes, optimal light scattering occurs at half the wavelength of light. Since the visible light spectrum ranges from about 400 to about 700 nm, the optimal particle size for opacification is from about 200 to about 350 nm. This is significantly larger than the particle size of stable black pigment-based inks used in consumer printing applications which generally comprise particles that are less than about 50 nm in size.
For cosmetic printing applications, it is known that the ink must be sufficiently opaque to cover and/or hide skin imperfections. However, it is difficult to formulate an opaque stable cosmetic ink which can be jetted due to the particle size and levels needed to achieve opacity and the gravitational settlement of the large and/or dense particles used to create such inks. For many years, the inkjet printing industry has attempted to create stable white inks that are compatible with inkjet printer cartridges and nozzle technology. However, even the most stable current white inks demonstrate particle settling over time if left undisturbed. Manufacturers of such inks recommend vigorous shaking on a daily or weekly basis to redisperse the particles and/or recommend that these inks are recirculated during use.
In addition to particle suspension stability, cosmetic inks should also be durable on a wide range of skin types. The particles in cosmetic inks must be deposited onto the skin and stay in place to be effective in covering skin imperfections. This is challenging because the surface properties of skin can vary on a single person and from person-to-person. It is known that the contact angle of water on a person's cheek can vary from 0-110 degrees due to different levels of sebum production and/or skin care technique variation from person to person. As a result, cosmetic inks will spread differently, dry at different rates, and have varying durability on the skin, and thus will provide varying coverage from person to person.
As such, there is a need for an opaque inkjet cosmetic ink composition that can be used on a wide range of skin types to hide and/or cover skin imperfections. In particular, there is a need for a cosmetic ink composition that comprises particles that can provide opacity and has a surface tension that will increase wettability and decrease dry time on skin, while still being stable.